Let's say we have a "seats" collection and seats can optionally belong to a user. If we query all the seats and eager load the users, but none of the seats actually belong to a user, Mongoid will still query the database for users where the id is

. I see something like this in the logs: {"find"=>"users", "filter"=>{"_id"=>{"$in"=>[nil]}}...

This change will skip the trip to the db if all the foreign keys are nil.

Submitted PR: https://github.com/mongodb/mongoid/pull/4621